Study On The Production And Degradation Characteristics Of VFAs In Co-digestion Process Of Food Waste And Sludge

With the acceleration of urbanization,the annual output of food waste and sludge increased at the rate of more than 10% per year,methane-fermentation is the main measure for food waste and sludge disposal and energy utilization,but,mono-digestion has the problems about imbalance of C/N,loss of trace elements and high concentrations of toxic substances or else,co-digestion can alleviate these problems effectively.However,excessive organic acids accumulated under high loading conditions is a key factor limit fermentation efficiency.This study is based on the phenomenon of volatile fatty acids(VFAs)accumulated in continuous experiments of co-digestion under mesophilic and thermophlic condition,analysis the effects of substrate's composition and temperature on the production characteristics and degradation kinetics of VFAs under co-digestion,from the perspective of the generation-degradation dynamic balance of VFAs,to reveal the reasons for the accumulation of typical VFAs during continuous experiments,the main conclusions are as follows:(1)The organic loading rate(OLR)is higher than 5.19 g-VS/L/d,VFAs begin to accumulate in the mesophilic or thermophilic co-digestion system,the co-digestion system operated stably when OLR continue increased to 11.13 g-VS/L/d,30.25 g-VS/L/d,the concentration of VFAs accumulated to 3.71 g-COD/L,4.07 g-COD/L under mesophilic and thermophlic system respectively,afterwards,serious VFAs accumulated in both system,and the digestion process was inhibited.Butyric acid was the dominant component of VFAs,accounting for 90% and 60-70% in the mesophilic and thermophlic systems respectively,propionic acid accumulated accounting for 15% in thermophlic system.The thermophlic conditions enhance the ammoniation process of organic nitrogen in the protein,provided alkalinity can buffer the rapidly accumulated VFAs in time,maintain the appropriate pH in the system,and ensure the thermophilic system can operate stably of the under higher organic loading rate.(2)Based on the characteristics that food waste is mainly composed by oil,vegetables,egg-meat,and staple foods,batch experiments were based on the production process characteristics of VFAs by different substrates shows that: butyric acid in VFAs is mainly derived from staple food components;propionic acid is mainly produced by hydrolysis and acidification of oil and egg-meat components with higher fat content.Further batch experiments of the fermentation characteristics of individual substrate(carbohydrates,cellulose,proteins,and oils)showed that the fermentation processes of different substrates had different VFAs production characteristics,verifying that carbohydrates are the main source of butyric acid,protein and grease is the main source of propionic acid.(3)Through the degradation kinetics experiments of typical VFAs(acetic acid,propionic acid,butyric acid,valeric acid),it was found that higher temperature has a significant role in promoting the methanation process of acetic acid and butyric acid under high loading conditions,under the same load conditions,maximum reaction rate is more than 2 times the mesophilic condition.However,the thermophlic condition has a significant inhibitory effect on the degradation of propionic acid and valeric acid.Therefore,under high temperature conditions,the inhibition of the fermentation process by propionic acid and valeric acid accumulation is higher than acetic acid and butyric acid,it was the main composition of VFAs that affected the methane yield.